CO2 Reforming of Methane in a Probe Reactor with a Thin Catalyst Layer

CO2 Reforming of Methane in a Probe Reactor with a Thin Catalyst Layer

Mohamed A. Al-Nakoua, Muftah H. El-Naas*

Chemical and Petroleum Engineering Department, UAE University

Abstract – Probe (annular) and channel reactors with thin catalyst layers are considered to be much more effective than conventional reactors and can offer several advantages such as excellent heat transfer characteristics and negligible intra- catalyst diffusion resistance. A probe reactor and a channel reactor were designed, fabricated and coated with thin layers of Ni/Al2O3 catalysts and tested for dry reforming of methane. The

recent catalytic research. Secondly, from an industrial perspective, the reaction produces syngas with a higher purity and a lower H2/CO ratio than steam reforming. The CO2 and CH4 are convertible to synthesis gas with H2/CO ratio about 1, which is suitable for specific synthesis processes such as alcohols by carbonylation [7,8]:

catalysts were initially prepared by the Sol-Gel method and

CH4 + CO2 2CO+ 2 H2 H0

= +247 kJ/mol(1)

calcined at different conditions. Several catalyst coats with different catalyst loading were tested at different operating conditions. High conversions were achieved on thin layer of Ni/Al2O3 catalysts and high coking resistance was obtained by

CO2

298

The great challenge in the industrial application of reforming of methane is deactivation and shattering of

addition of Cr, Ba and La2O3. The experimental results also showed that as CO2/CH4 ratio increased the coking rate decreased. Low pressure drop and low temperature gradient

catalysts from carbon deposition via CO

disproportionation, Reaction 2 and methane decomposition, Reaction 3.

along the length of the reactor were attained. The catalyst effectiveness factor was about 1 and the values of activation energy indicated the reactions did not have any diffusion

2CO Cs + CO2

H0

298

k = -171 kJ/mol (2)

limitations.

Keywords:CO2 Utilization; Probe Reactor; Channel Reactor; Thin Layers; Sol-gel; Process Intensification.

1. INTRODUCTION

   Intensified catalytic reactors (ICR) have several advantages compared to conventional packed bed, slurry, or tubular reactors. Besides the reduction in size, ICR have negligible mass and heat transfer resistances, due to the use of a thin catalyst layer coated on the reactor surface, which reduces mass and heat transfer restrictions compared with pellet catalysts and can improve the effectiveness factor. It is reported that the functioning layer of catalyst pellets in a conventional reforming tube is only about 50 µm thick [1]. The vital advantage of catalytic plate reactor concept would be that thin catalyst films would have much less resistance to transport processes than traditional catalyst pellets. Hence catalyst effectiveness factors would be near unity. There have been several recent studies of the application of plate reactors [2-4]; CO2 reforming of CH4 has attracted interest from both industrial and environmental perspectives [5,6]. The CO2 reforming of methane has two major advantages over steam reforming of methane or partial oxidation of methane. Firstly, from an environmental point view, the two most abundant carbon containing greenhouse gases, methane and carbon dioxide, can be utilized effectively in this reaction and converted into useful chemical products. This is an important area of

   CH4 Cs + 2 H2 H0298 k =+75 kJ/mol (3)

   Relatively speaking, Reaction 2 is favored at low temperature and low pressure, while Reaction 3 is favored at high temperature and low pressure. Therefore, substantial number of publications were produced on different strategies implemented to minimize the amount of coke[5-7, 9-13]; optimizing the operating conditions (feed ratio), use of alternative catalysts (additives), control of surface reactions and take advantage of catalyst characterizations to optimize the preparation method and reduction temperature.

   Preparation techniques are crucial in developing an active, selective, stable and durable catalyst. Although the sol-gel technique has been adopted for stainless steel substrate and micro-channel reactors coatings [2,3], there have been limited studies on the characterization and testing of this catalyst coats prepared by the Sol-Gel method. Li and Wang [7] investigated the preparation and characterization of Sol-gel catalysts. However, their study focused on using catalyst powder in a fixed bed reactor for CO2 reforming reactions. To the best of the authors knowledge, there have been no reports in the open literature on the testing of Sol-Gel coats for important potential applications such as dry reforming of methane. The objective of the present study, therefore, is to examine the performance of dry reforming of methane reaction onto a probe and channel reactor coated by thin layers of catalyst via a sol gel method. The activity and stability of

   the CO2 reforming over Ni/Al2O3 and Ni-Cr-Ba/La2O3- Al2O3 catalysts were investigated.

2. EXPERIMENTAL SETUP

   1. Annular or Probe Reactor

      The probe reactor used in the study was fabricated from 316 stainless steel tubing (12.7 mm OD, 9.5 mm ID) with a length of 500 mm. An inner steel tube (6.4 mm OD), sealed at one end, was centrally installed via the exit tee fitting, inside the outer tube which formed a narrow (1.55 mm) annulus with 9.5 mm OD and 6.4 mm ID. The inner tube had small spot welds along its length which protruded from the tube and were a sliding fit inside the outer tube. This ensured that the inner tube was positioned centrally inside the outer tube. Approximately 150 mm of the outside of the inner tube was coated with catalyst to give a coated area of approximately 30 cm2 and an annulus volume of 5.88 cm3 around the catalyst. Thermocouples were placed inside the inner tube to measure the temperature inside the reactor. The main body of the reactor was held axially within a three zone furnace such that the middle of the coated area was approximately mid- way along the furnace length. The entrance and exit tee fittings were held outside the furnace and lagged with insulation.

   2. Catalysts

      Catalysts were prepared by dispersing Disperal alumina supplied by Sasol, Germany [14] in 1% nitric acid and adding the appropriate nickel and other additives as nitrates to give composition of calcined catalysts in wt%; Ni(49%)/Al2O3(51%) (Catalyst A) and Ni(33%)-Cr(5.6%)- Ba(11%)/ La2O3(19%)-Al2O3(31.4) (Catalyst B),

      respectively. The inner tube of the reactor described in

      The reactants were preheated in the furnace before entering the reactor.

3. RESULTS AND DISCUSSION

3.1 Thermodynamic analysis

Dry reforming of methane (Reaction 1) is an important reaction that produces syngas with a higher purity and a lower H2/CO ratio than steam reforming. It also utilizes CO2 as a gaseous pollutant that is well known to contribute to global warming.

A thermodynamic analysis was carried out for Reaction (1) to determine the equilibrium composition at different temperatures and to estimate the heat of reaction as a function of temperature. The analysis was conducted using HSC Chemistry, a chemical reaction and equilibrium software. For a fixed temperature and pressure, the number of moles present at equilibrium for any species can be determined using the Gibbs free energy minimization method.

Reaction (1) is highly endothermic and becomes spontaneous for temperatures beyond 640 °C as shown in Figure 1. The catalytic eaction is usually carried out in the temperature range of 600 to 800 °C, where the heat of reaction is almost constant with temperature as shown in the figure. The equilibrium composition of the two reactants (CH4 and CO2) and two products (CO and H2) is presented in Figure 2 at a pressure of 1 Bar. The figure shows that the reaction commences at about 400 oC and reaches completion at about 1000 °C. It is worth noting here that both reactants and both products appear on the same line as shown in the figure and as indicated by the initial equimolar stoichiometry of the reaction.

Section 2.1 was pre-treated before coating to improve adhesion. Approximately 150 mm from the sealed end of the tube was roughened using coarse emery paper and then degreased by dipping in a measuring cylinder containing acetone. The tube then weighed using 4-decimal place analytical balance. This 150 mm roughened part of the tube was coated by dipping into a measuring cylinder containing the sol-gel. The coated area was 150 mm long and marked with a PTFE tape, which was also used to prevent the coating of the seals tip end. The probe was submerged in the sol gel for couple of seconds, lifted out and dried at 100

°C. Multiple dips were used to build up the desired

200

150

G (kJ/kmol)

100

50

0

-50

-100

-150

262

			G H

260

258

H (kJ/kmol)

256

254

252

250

248

246

244

0 200 400 600 800 1000

Temperature (oC)

catalyst weight and finally calcined at 400 °C. The reactor described in Section 2.1 was coated with catalyst by forcing the sol-gel through the channel, blowing out the surplus and air-drying at 100 °C. Multiple coats were used to build up the desired catalyst weight and the catalyst then calcined at 400 °C. The reactors blocks were light enough to be weighed to +/-1 mg, giving accuracy in the mass of catalyst of +/- 1% or better.

1. Experimental Procedure

After calcination, the reforming catalysts were reduced in situ in a stream of hydrogen at 600°C for at least 2 hours and a small flow of hydrogen was maintained throughout the experiments to reduce carbon lay down, Reactions 3.

Figure 1. Heat of reaction and Gibbs free energy for Reaction (1) at different temperatures at 1 Bar .

2.0

CO, H

Equilbrium Composition (kmol)

1.8

2

CH4, CO2

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0 200 400 600 800 1000 1200

Temperature (oC)

1.0

0.9

0.8

0.7

Conversion

0.6

0.5

0.4

0.3

0.2

0.1

0.0

CO2

CH4

Figure 2.Equilibrium composition for Reaction (1) at different temperature and 1 Bar.

Since the dry reforming reaction is a gaseous reaction with volume increase, it is highly affected by the operating pressure. High pressure tends to hamper the progress of the reaction. On the other hand, low pressure favors the forward reaction and leads to reaction completion at a much lower temperature as shown in Figure 3. Although

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Time (h)

Figure 4.Methane and CO2 conversion vs. time over Catalyst A; catalystloading 15.23 mg/cm2 in Probe reactor at 700 oC furnace temperature

Table 1.Effect of Feed Ratio on the Rate of Carbon Deposition and Product Ratio forCatalyst B

the probe reactor was not tested at pressures lower than atmospheric, the thermodynamic analysis shown in Figure 3 suggests that low pressure may be more suitable for dry

CO2/CH4

Feed ratio

Mole carbon deposited/ mole carbon converted

Coking rate [g- carbon/h]

H2/CO

Product

ratio

CH4

Conversion %

reforming and may favor Reaction 1 over Reaction 2 and

hence lead to less carbon deposition.

1 0.022 1.05 0.81 89-93

2.0

Equilbirium Composition (kmol)

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

1 Bar

5 Bar

0.01 Bar

1.17 0.0084 0.4297 0.79 87

1.34 0.0041 0.22761 0.74 80-90

3.3 Reaction Kinetics

A previous study [15] confirmed that replacing steam with CO2 in the reforming reaction has no drastic influence on the mechanism. However, the experimental results over Catalyst B suggested that the reaction order with respect to methane is approaching first order and the reaction order

0.0

0 200 400 600 800 1000 1200

Temperature (oC)

Figure 3.Equilibrium compositions for CO and H2 of Reaction (1) at different temperatures and different pressures.

3.2 Catalyst Activity and Feed Ratios

The catalyst film gave about 80% methane and carbon dioxide conversion at atmospheric pressure as shown in Figure 4. The methane flow rate was 0.58 mol/hr with 1:1 molar CH4:CO2 ratio. From Reaction 1, it can be seen that the stoichiometric requirement for CO2/CH4 ratio is 1 and from an industrial point view, it may be desirable to operate with CO2/CH4 ratios near unity. Previous studies [9, 10] recommended that CO2/CH4 ratio be above unity to prevent carbon deposition. However, the results showed that as CO2/CH4 ratio increased the coking rate decreased as well as H2/CO product ratio 1. The results of each test over Catalyst B were summarized in Table 1.

with respect to CO2 being closer to zero order. These results are in agreement with those of [15, 16] who reported a zero order dependence with respect to CO2, as well as first-order dependence with respect to methane, in CO2 reforming over Ni-based catalyst. The rate of CO2 reforming was described by applying Langmuir- Hinshelwood rate equations [17].

1. Diffusion Limitations

   In catalytic plate reactors, small channels coated with a thin catalyst film offer short diffusion and conduction path lengths for rapid mass, and heat transfer. This results in minimal intra-catalyst diffusion limitations and thus high catalyst utilization (effectiveness factor 1). However, results showed that increase of the catalyst loading in the plate reactors results in enhancement of methane and carbon dioxide conversion under the same reaction conditions, see Figure 5. The catalyst thickness is estimated to be up to 50 µm, assuming that the layer is uniformly deposited and has a porosity of 35%. At such thickness, internal mass and heat transfer resistances are considered to

   be negligible. The apparent activation energy was varied from 110 to 85 kJ/mol when catalyst loading increased from 2 mg/cm2 (7.7 µm) to 6 mg/cm2 (23.1 µm), based on the density of Catalyst B estimated at 2.6 g/cc. These activation energy values are within the range reported by other researchers [16, 18, 19] who used Ni-based catalysts. It can be seen that the presence of promoters in Ni/Al2O3 catalyst film have minor effects on activation energy magnitudes. As a rule of thumb, chemical reaction is rate limiting if the apparent activation energy is greaterthan40 kJ/mol. If it is in the range of 12-15 kJ/mol or lower, then the transport processes are assuming a greater degree of control over the reaction [20]. This means that for the range of catalyst loadings examined, the reaction apparent activation energy values are too high for any diffusion limitation.

   90

   Methane Conversion (%)

   80

   70

   60

   50

   40

   30

   20 600 oC

   650 oC

   10 657 oC

   0

   0 2 4 6 8 10 12 14 16 18

   Catalyst Loading (mg/cm2)

   Figure 5.Methane conversion as a function of catalyst loading for different reactor temperatures.

   CONCLUSIONS

   • It has been demonstrated that sol gel method facilitates catalyst preparation. The sol-gels can be prepared to have good rheological properties for coating onto stainless steel substrate which after calcining form an adherent thin catalyst layer with good metal dispersion.

   • The catalyst film gave about 80% methane and carbon dioxide conversion.

   • Carbon deposition rate was high on Ni/Al2O3 (Catalyst

     1. and reduced by the addition of Ba, Cr and La2O3 as well as CO2/CH4 ratio increased the coking rate decreased.

• Probe (annular) reactor is more convenient for rapid activity tests.

• Low temperature gradient along the length of the channel reactor was attained compared to probe reactor.

• Low pressure drop along the length of the reactors were achieved.

• The values of activation energy indicate the reaction is free from any diffusion limitation effect.

ACKNOWLEDGEMENT

The authors would like to acknowledge the financial support provided by the Research Affairs at the UAE University.

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